Carrier aggregation is one of the new features being discussed for the next generation of Long Term Evolution (LTE) systems, which is being standardized as part of LTE Release 10 (known as LTE-Advanced). LTE Rel 8 currently supports bandwidths up to 20 MHz. In LTE-Advanced, bandwidths up to 100 MHz will be supported. The very high data rates contemplated for LTE-Advanced will require an expansion of the transmission bandwidth. In order to maintain backward compatibility with LTE Rel-8 user terminals, the available spectrum is divided into Rel-8 compatible chunks called component carriers. Carrier aggregation enables the needed bandwidth expansion by allowing user terminals to transmit data over multiple component carriers comprising up to 100 MHz of spectrum. Carrier aggregation also ensures efficient use of a wide carrier for legacy terminals by making it possible for legacy terminals to be scheduled in all parts of the wideband LTE-Advanced carrier.
The number of aggregated component carriers, as well as the bandwidth of the individual component carrier, may be different for Uplink (UL) and Downlink (DL). A symmetric configuration refers to the case where the number of component carriers in downlink and uplink is the same. An asymmetric configuration refers to the case where the number of component carriers is different. The number of component carriers configured for a geographic cell area may be different from the number of component carriers seen by a terminal. A user terminal, for example, may support more downlink component carriers than uplink component carriers, even though in the geographic cell area the same number of uplink and downlink component carriers is offered by the network.
One consideration for carrier aggregation is how to transmit control signaling from the user terminal on the uplink from the user terminal. Uplink control signaling may include acknowledgement (ACK) signaling for hybrid automatic repeat request (HARQ) protocols, channel state and quality information (CSI, CQI) reporting for downlink scheduling, and scheduling requests (SRs) indicating that the user terminal needs uplink resources for uplink data transmissions. One solution is to transmit the uplink control information on multiple uplink component carriers associated with different downlink component carriers. However, this option is likely to result in higher user terminal power consumption and a dependency on specific user terminal capabilities. It may also create implementation issues due to inter-modulation products, and may lead to generally higher complexity for implementation and testing.